1. Field of the Invention
The present invention is broadly concerned with methods and apparatus used in the processing of biomass to yield torrefied and/or carbonized (i.e., charcoal) biomass. More particularly, the invention is concerned with such methods and apparatus whereby starting biomass is initially dried and then torrefied in a specialized indirect torrefaction reactor, with the combustible gases evolved from the biomass during torrefaction being used as a source of fuel for the initial biomass drying step. Preferably, at least a portion or all of the torrefied product may be subsequently carbonized in a separate indirect carbonization reactor, with the combustible gases evolved from carbonization also being used as a source of fuel. The processes of the invention are characterized by high energy efficiency, and in some forms the entire thermal energy required for steady-state processing is derived from the evolved combustible gases.
2. Description of the Prior Art
Biomass as understood in the art and as used herein refers to a biological material derived from living, or recently living organisms. In the context of biomass for energy, this is often used to mean plant-based materials, but biomass can equally apply to both animal and vegetable-derived materials. Biomass is carbon-based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, and often with other atoms, including alkali, alkaline earth, and heavy metals. Plant-based biomass is normally lignocellulo sic in nature and can be derived from a variety of sources: wood, such as forest waste, arboricultural activities, and wood processing; agricultural residues, such as corn stover and rice straw; grasses, such as switchgrass and miscanthus. Other biomass sources include food waste from food and drink manufacture, preparation, and processing, or industrial waste, municipal solid waste, and animal waste. Biomass as-received generally has a moisture content of 5-80% by weight.
Techniques have been developed in the past for processing biomass to obtain useful fuels. Generally, the native biomass is preliminarily pre-sized, dried and sized, and is thereafter thermally treated to obtain various end products, including torrefied biomass and carbonized (charcoal) biomass. Torrefaction involves thermal processing to evolve combustible organic gases, particularly volatile organic compounds (VOCs). However, the torrefied product still contains heavy VOCs and, if used as a fuel, will have a tendency to “smoke.” Carbonization removes most of the remaining VOCs in the torrefied biomass, leaving a residue which is essentially free of smoke-producing compounds and is composed essentially of fixed carbon.
Generally, attempts have been made to carry out many or all stages of the biomass processing in a direct-fired single reactor. This has proved to be problematical because of low production rates and the fact that a single reactor cannot provide the optimum conditions for drying, torrefaction, and carbonization. Moreover, the throughput of single-reactor systems is relatively low, given that most single carbonization systems are batch systems.